1. Field of Invention
The present invention pertains to the pumping of fluids into a wellbore, and particularly to the pumping of fluids during a gravel pack operation.
2. Related Art
It is often desired to place sand or other filtering medium between a sand screen and the wellbore in wells having poorly or loosely consolidated production formations. Without the presence of such filtering media, screens can become plugged or production equipment can be damaged by fines (small particulates of the formation matrix) or formation sand produced with the production fluids.
The sand is typically conveyed entrained in a fluid. The entrained sand is generally referred to as “gravel” and the gravel and conveyance fluid combination is generally referred to as a “gravel slurry.” The gravel slurry is normally pumped into the well through tubing until it reaches a crossover, at which point the slurry enters the annulus between the sand screen and the wellbore or casing. As the conveyance fluid is either lost to the formation or returns to the surface, the gravel settles out, packing the annulus.
A gravel pack operation usually requires the pumping of more than one type of fluid. The volume of gravel slurry pumped is generally calculated based on the anticipated volume necessary to fill the annular space with gravel. Spacer fluids are generally placed ahead of and behind the gravel slurry to provide for fluid isolation and separation during the gravel pack operation. Other fluids, such as brine or drilling fluid “mud”, may be pumped behind the gravel slurry as displacement fluid to force the gravel slurry into the proper location in the well. Thus, different fluids may be present or introduced into the well both ahead of and behind the gravel slurry.
Because the different fluids have different physical properties, such as density and viscosity, they can sometimes mix, or one fluid may flow past the other. Those undesired effects may be exacerbated in horizontal or highly deviated portions of the wellbore or if fluids are traveling down the wellbore at low velocities. That may lead to a reduction in displacement efficiency, preventing a particular fluid from reaching its intended location in the well. That, in turn, may lead to an incomplete gravel pack operation.